The invention relates to a sliding member, and more particularly to a sliding member having a thin coating of hard carbon, such as diamond-like carbon, to show excellent low-friction characteristics and durability in the presence of a specific lubricant. The invention also relates to a process for producing the sliding member.
Global environmental problems, such as global warming and ozone layer destruction, are coming to the fore. The global warming is significantly affected by CO2 emission, and the setting of CO2 emission standards to reduce CO2 emission has become a big concern to each country. In order to reduce CO2 emission, it is important to improve vehicle fuel efficiency. The reduction of friction in a vehicle engine is thus desired to obtain a direct improvement in fuel efficiency.
There are some conceivable ways to reduce the engine friction. One way to reduce the engine friction is to provide engine sliding members with lower friction coefficients and higher wear resistance under extreme friction/wear conditions. For example, it is proposed to apply hard carbon coating materials to the cam follower portions of engine sliding members (such as a valve lifter and a lifter shim) and to use a so-called roller rocker arm equipped with a roller needle bearing. As it has been proved that a diamond-like carbon (DLC) coating shows a lower friction coefficient in the air than those of titanium nitride (TiN) film and chromium nitride (CrN) film, the DLC coating is expected to be useful for the engine sliding members. Another way to reduce the engine friction is to improve the properties of a lubricating oil applied to the sliding members. It is proposed to lower lubricating oil viscosity so as to reduce viscous resistance in hydrodynamic lubrication regions and agitation resistance in mechanical sliding portions. It is also proposed to provide a lubricating oil blended with a suitable friction modifier and other additives so as to reduce engine friction losses under mixed lubrication conditions and boundary lubrication conditions. Many studies have been made on various friction modifiers including organomolybdenum compounds e.g. molybdenum dithiocarbamate (MoDTC) and molybdenum dithiophosphate (MoDTP), and the lubricating oil containing such an organomolybdenum friction modifier is proved to be effective in reducing the friction between steel sliding members in the early stages of use.
The low-friction characteristics of the DLC coating and the friction modifying properties of the organomolybdenum compound are reported in Japan Tribology Congress 1999. 5, Tokyo, Proceeding Page 11-12, KANO et al. and World Tribology Congress 2001. 9, Vienna, Proceeding Page 342, KANO et al.